Add a migration for the dataloader loops (#17125)

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

src/lightning/pytorch/utilities/migration/migration.py

@@ -50,6 +50,7 @@ def _migration_index() -> Dict[str, List[Callable[[_CHECKPOINT], _CHECKPOINT]]]:
             _drop_apex_amp_state,
             _migrate_loop_structure_after_tbptt_removal,
             _migrate_loop_structure_after_optimizer_loop_removal,
+            _migrate_loop_structure_after_dataloader_loop_removal,
         ],
     }
 
@@ -236,7 +237,8 @@ def _migrate_loop_structure_after_tbptt_removal(checkpoint: _CHECKPOINT) -> _CHE
     """
     if "loops" not in checkpoint:
         return checkpoint
-
+    if "fit_loop" not in checkpoint["loops"]:
+        return checkpoint
     fit_loop = checkpoint["loops"]["fit_loop"]
 
     # remap `x.batch_loop.y` to `x.y`
@@ -273,8 +275,10 @@ def _migrate_loop_structure_after_optimizer_loop_removal(checkpoint: _CHECKPOINT
     """
     if "loops" not in checkpoint:
         return checkpoint
-
+    if "fit_loop" not in checkpoint["loops"]:
+        return checkpoint
     fit_loop = checkpoint["loops"]["fit_loop"]
+
     # optimizer_position is no longer used
     if "epoch_loop.optimizer_loop.optim_progress" in fit_loop:
         fit_loop["epoch_loop.optimizer_loop.optim_progress"].pop("optimizer_position", None)
@@ -291,3 +295,25 @@ def _migrate_loop_structure_after_optimizer_loop_removal(checkpoint: _CHECKPOINT
             "epoch_loop.manual_loop.optim_step_progress"
         )
     return checkpoint
+
+
+def _migrate_loop_structure_after_dataloader_loop_removal(checkpoint: _CHECKPOINT) -> _CHECKPOINT:
+    """The dataloader loops (``_DataLoaderLoop``, ``_PredictionLoop`, and ``_EvaluationLoop``) were flattened into
+    the ``_EvaluationEpochLoop`` (now ``_EvaluationLoop``) and ``_PredictionEpochLoop`` (now ``_PredictionLoop``).
+
+    Version: 2.0.0
+    Commit: ec4f592ecfe238edd83185f6c6905fb1e2406d61
+    PR: #16726
+    """
+    if "loops" not in checkpoint:
+        return checkpoint
+    loops = checkpoint["loops"]
+    for loop_key in ("predict_loop", "validate_loop", "test_loop"):
+        if loop_key not in loops:
+            continue
+        loop = loops[loop_key]
+        loop.pop("dataloader_progress", None)  # no longer used
+        epoch_loop_key = "epoch_loop."
+        epoch_loop_dict = {k[len(epoch_loop_key) :]: loop.pop(k) for k in list(loop) if k.startswith(epoch_loop_key)}
+        loop.update(epoch_loop_dict)
+    return checkpoint

tests/tests_pytorch/utilities/migration/test_migration.py

@@ -227,3 +227,52 @@ def test_migrate_loop_structure_after_optimizer_loop_removal():
             "epoch_loop.manual_optimization.optim_step_progress": optim_progress_manual,
         }
     }
+
+
+def test_migrate_loop_structure_after_dataloader_loop_removal():
+    """Test the loop state migration after the dataloader loops were removed in 2.0.0."""
+    old_dataloader_loop_state_dict = {
+        "state_dict": {},
+        "dataloader_progress": {"total": {"ready": 0, "completed": 0}, "current": {"ready": 0, "completed": 0}},
+        "epoch_loop.state_dict": {},
+        "epoch_loop.batch_progress": {
+            "total": {"ready": 123, "started": 0, "processed": 0, "completed": 0},
+            "current": {"ready": 0, "started": 0, "processed": 0, "completed": 0},
+            "is_last_batch": False,
+        },
+    }
+    old_checkpoint = {
+        "loops": {
+            "predict_loop": old_dataloader_loop_state_dict,
+            "validate_loop": dict(old_dataloader_loop_state_dict),  # copy
+            "test_loop": dict(old_dataloader_loop_state_dict),  # copy
+        }
+    }
+    _set_version(old_checkpoint, "1.9.0")  # pretend a checkpoint prior to 2.0.0
+    updated_checkpoint, _ = migrate_checkpoint(old_checkpoint.copy(), target_version="2.0.0")
+    assert updated_checkpoint["loops"] == {
+        "predict_loop": {
+            "batch_progress": {
+                "current": {"completed": 0, "processed": 0, "ready": 0, "started": 0},
+                "is_last_batch": False,
+                "total": {"completed": 0, "processed": 0, "ready": 123, "started": 0},
+            },
+            "state_dict": {},
+        },
+        "test_loop": {
+            "batch_progress": {
+                "current": {"completed": 0, "processed": 0, "ready": 0, "started": 0},
+                "is_last_batch": False,
+                "total": {"completed": 0, "processed": 0, "ready": 123, "started": 0},
+            },
+            "state_dict": {},
+        },
+        "validate_loop": {
+            "batch_progress": {
+                "current": {"completed": 0, "processed": 0, "ready": 0, "started": 0},
+                "is_last_batch": False,
+                "total": {"completed": 0, "processed": 0, "ready": 123, "started": 0},
+            },
+            "state_dict": {},
+        },
+    }